silyl Group 10 Metal Complexes - ACS Publications - American

Addition of the silane (HSiMe2)C2B10H10(PR2) (R ) Me (2a), OEt (2b)) to (PPh3)2Pt(CH2- ... Pt(CabP,Si)2 (4a,b; CabP,Si ) η2-[(SiMe2)(PR2)C2B10H10-...
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Organometallics 2004, 23, 203-214

203

Synthesis, Structure, and DFT Calculation of (Phosphino-o-carboranyl)silyl Group 10 Metal Complexes: Formation of Stable trans-Bis(P,Si-chelate)metal Complexes Young-Joo Lee,† Jong-Dae Lee,† Sung-Joon Kim,† Samrok Keum,† Jaejung Ko,*,† Il-Hwan Suh,† Minserk Cheong,‡ and Sang Ook Kang*,†,§ Department of Chemistry, Korea University, 208 Seochang, Chochiwon, Chung-nam 339-700, Korea, and Department of Chemistry and Research Institute for Basic Sciences, Kyung Hee University, Seoul 130-701, Korea Received August 14, 2003

Addition of the silane (HSiMe2)C2B10H10(PR2) (R ) Me (2a), OEt (2b)) to (PPh3)2Pt(CH2CH2) (3) affords the trans-bis(chelates) Pt(CabP,Si)2 (4a,b; CabP,Si ) η2-[(SiMe2)(PR2)C2B10H10P,Si]) in high yield; the same product was also formed from Pt(cod)2 (9), as confirmed by NMR spectroscopy (1H, 31P). Using Pd2(dba)3 (6), the analogue trans-bis(chelate) Pd(CabP,Si)2 (7a) was obtained as a mixture of trans and cis isomers in which the former predominates, as established by NMR spectroscopy (1H, 31P). Thus, a series of kinetically stabilized transbis(chelate) metal complexes, trans-(CabP,Si)2M (M ) Pt (4a,b), Pd (7a)), bearing bulky o-carboranylphosphine tethers, were synthesized from the reaction of phosphinosilanes (2) with d10 metal complexes. In the presence of dimethyl acetylenedicarboxylate (DMAD), the trans isomer 4a,b thermally rearranges to the thermodynamically favored cis isomer cis(CabP,Si)2Pt (5a,b). In addition, the oxidative addition of the Si-H bond to the sterically bulky diphenylphosphino silane (HSiMe2)C2B10H10(PPh2) (2c) by 3 can be controlled to produce the mono(chelate) species (CabP,Si)Pt(H)(PPh3) (10a). The related mono(chelate) product (CabP,Si)Pt(H)(CabP) (10b; CabP ) (PPh2)C2B10H11) results from the oxidative addition of 2c by 3, in which one PPh3 ligand is displaced by CabP. The structures of compounds 4a, 5a,c, 8c, and 10a,b were determined using single-crystal X-ray crystallography. Introduction (Phosphinoalkyl)silanes are used as chelate ligands in a wide range of oxidative addition reaction processes with transition metals1 and have been studied in considerable detail, principally to obtain a better understanding of industrially important metal-catalyzed reactions such as hydrosilylation.2 Such molecules undergo oxidative addition at low-valent transitionmetal centers in which Si-M bond formation is accompanied by chelation with phosphine donors, affording ((phosphinoalkyl)silyl)metal complexes. Although many examples of bis(chelate) metal complexes with a cis arrangement of the (phosphinoalkyl)silyl ligands in a typical square-planar M(II) (M ) Pd, Pt) environment have been synthesized3 and evaluated in the aforemen-

tioned context, comparatively few trans-bis(chelates)3c have received scrutiny, principally because of their inaccessibility. Recent reports of unusually stable cyclic bis(o-carboranylsilyl)metal complexes4 appear to imply that the properties of these complexes, such as their chelation, rigid conformation, and o-carboranyl ligand backbones might be ideal for the kinetic stabilization of metal silyl intermediates in the double-silylation process.5 Therefore, to increase the kinetic stability of this type of ligand framework, we have synthesized (phosphinoalkyl)silane precursors with a two-carbon skeleton connecting the silicon and phosphorus atoms that is part of a bulky o-carboranyl unit. These new (phosphino-o-carboranyl)silanes were used in this study to isolate a series of trans-bis(chelate)metal complexes, which are formed by “chelate-assisted” oxidative addi-

†

Korea University. Kyung Hee University. Tel: +82-41-860-1334. Fax: +82-41-867-5396. E-mail: sangok@ korea.ac.kr. (1) (a) Okazaki, M.; Ohshitanai, S.; Iwata, M.; Tobita, H.; Ogino, H. Coord. Chem. Rev. 2002, 226, 167. (b) Brost, R. D.; Bruce, G. C.; Joslin, F. L.; Stobart, S. R. Organometallics 1997, 16, 5669. (c) Joslin, F. L.; Stobart, S. R. Inorg. Chem. 1993, 32, 2221. (d) Ang, H. G.; Chang, B.; Kwik, W. L. J. Chem. Soc., Dalton Trans. 1992, 2161. (e) Joslin, F. L.; Stobart, S. R. J. Chem. Soc., Chem. Commun. 1989, 504. (f) HolmesSmith, R. D.; Stobart, S. R.; Vefghi, R.; Zaworotko, M. J.; Jochem, K.; Cameron, T. S. J. Chem. Soc., Dalton Trans. 1987, 969. (g) Auburn, M. J.; Stobart, S. R. Inorg. Chem. 1985, 24, 318. (h) Auburn, M. J.; Holmes-Smith, R. D.; Stobart, S. R. J. Am. Chem. Soc. 1984, 106, 1314. (2) (a) Speier, J. L. Adv. Organomet. Chem. 1979, 407. (b) Chalk, A. J.; Harrod, J. F. J. Am. Chem. Soc. 1965, 87, 16. ‡ §

(3) (a) Gossage, R. A.; McLennan, G. D.; Stobart, S. R. Inorg. Chem. 1996, 35, 1729. (b) Grundy, S. L.; Holmes-Smith, R. D.; Stobart, S. R.; Williams, M. A. Inorg. Chem. 1991, 30, 3333. (c) Schubert, U.; Muller, C. J. Organomet. Chem. 1989, 373, 165. (d) Holmes-Smith, R. D.; Stobart, S. R.; Cameron, T. S.; Jochem, K. J. Chem. Soc., Chem. Commun. 1981, 937. (4) (a) Kang, Y.; Ko, J.; Kang, S. O. Organometallics 2000, 19, 1216. (b) Kang, Y.; Ko, J.; Kang, S. O. Organometallics 1999, 18, 1818. (c) Kang, Y.; Lee, J.; Kong, Y. K.; Ko, J.; Kang, S. O. Chem. Commun. 1998, 2343. (5) (a) Uchimaru, Y.; Lautenschlager, H. J.; Wynd, A. J.; Tanaka, M.; Goto, M. Organometallics 1992, 11, 2639. (b) Tanaka, M.; Uchimaru, Y.; Lautenschlager, H. J. Organometallics 1991, 10, 16. (c) Eaborn, C.; Metham, T. N.; Pidcock, A. J. Organomet. Chem. 1973, 63, 107.

10.1021/om034112l CCC: $27.50 © 2004 American Chemical Society Publication on Web 12/12/2003

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Organometallics, Vol. 23, No. 2, 2004 Scheme 1. Selective Formation of trans-Bis(P,Si-chelate)metal Complexes 4a,b and 7a

Lee et al. Scheme 3. Preparation of Trans Platinum Metal Chelates 4a,ba

a

Scheme 2. Synthesis of (Phosphino-o-carboranyl)silane 2a

a Conditions: (i) (a) BunLi, THF, -78 °C; (b) R2PCl (R ) Me (a), OEt (b), Ph (c)), THF, -78 °C; (ii) (a) BunLi, THF, -78 °C; (b) Me2SiHCl, THF, -78 °C.

tion at palladium or platinum, as shown in Scheme 1. The early results of this study have previously been communicated.6 Results and Discussions Preparation of (Phosphino-o-carboranyl)silanes (2). The (phosphino-o-carboranyl)silanes (2) were synthesized according to Scheme 2. A synthetic route to 2 is provided by the action of 1-lithio-o-carborane on chlorodialkylphosphine. This route affords 1-(dialkylphosphino)-o-carborane (1) in good yield as a colorless solid; then lithiation of 1 at low temperature followed by reaction with dimethylchlorosilane produces 2 (Scheme 2). A wide variety of (phosphino-o-carboranyl)silanes (2) with a Si-H bond were synthesized in this way as ligand precursors. Each product (2a-c) was recovered as a colorless, rather waxy solid in ca. 82-92% yield; survival of the silyl (i.e. Si-H) functionality was obvious from the strong IR absorptions of these products near 2162-2170 cm-1 and from the observation of 1H NMR signals at δ 4.32 (2a), 4.31 (2b), and 4.50 (2c) (δ(Si-H)) as multiplets that were integrated correctly vs alkyl or aryl protons. Further characterization was provided by the 13C NMR (alkyl or aryl carbons) and 31P NMR chemical shifts (Table 1). (6) Lee, Y.-J.; Bae, J.-Y.; Kim, S.-J.; Ko, J.; Choi, M.-G.; Kang, S. O. Organometallics 2000, 19, 5546.

Conditions: (i) (PPh3)2Pt(C2H4) (3), toluene, 25 °C.

Synthesis of [η2-(Phosphino-o-carboranyl)silyl]metal Complexes (CabP,Si)2M (CabP,Si ) η2-(PR2)(SiMe2)C2B10H10-P,Si; M ) Pt (4, 5), Pd (7, 8)). Metal chelates with η2-[(PR2)(SiMe2)C2B10H10-P,Si] were synthesized using the reaction of coordinatively unsaturated metal complexes with the (phosphinoalkyl)silanes 2 via coordination of the phosphine moiety and the oxidative addition of the Si-H bond. When 2 equiv of the (phosphinoalkyl)silanes 2a,b was treated with 1 equiv of (PPh3)2Pt(C2H4) (3) in toluene at 25 °C, an immediate color change from pale yellow to deep yellow occurred (Scheme 3). Standard workup and crystallization from toluenepentane gave (CabP,Si)2Pt (4a,b) in 73-76% yield as a yellow, crystalline solid which was stable in air and stable during brief heating to 100-110 °C. The unusual thermal stability of 4a,b may be related to the advantageous properties of the o-carboranyl unit, including both its electronic and steric effects. Compounds 4a,b were soluble in toluene, THF, and chloroform and were characterized using 1H, 13C, and 31P NMR and elemental analysis. A trans arrangement of the (phosphinoalkyl)silyl ligands in a typical square-planar Pt(II) environment is proposed here on the basis of the 1JPt-P values, which are in the range 2702-4049 Hz for 4a,b.7 In particular, the 1H NMR chemical shift of δ 1.87 for 4a as distorted triplets (2JH-P ) 3 Hz) resembles the values reported for the typical virtual coupling in transPt(PMe2R)2X2 (R ) Me, Ph) complexes.8 This conclusion was substantiated for complex 4a by using a singlecrystal X-ray study. This molecule has a square-planar coordination around the Pt center with two mutually trans SiMe2- units, as shown in Figure 1. The Pt-Si bond is elongated (2.408(1) Å) by the large influence of the SiMe2 ligand in the trans position. The reaction in Scheme 2 proceeds with high stereoselectivity, resulting preferentially in the formation of the trans isomer. This is unusual, as the chelate-assisted oxidative addition of (phosphinoalkyl)silanes with d10 metal complexes usually occurs with cis stereochemistry3b,d due to the (7) (a) Kalt, D.; Schubert, U. Inorg. Chim. Acta 2000, 306, 211. (b) Kim, Y.-J.; Park, J.-I.; Lee, S.-C.; Osakada, K.; Tanabe, M.; Choi, J.C.; Koizumi, T.-A.; Yamamoto, T. Organometallics 1999, 18, 1349. (8) (a) Yamashita, H.; Tanaka, M.; Goto, M. Organometallics 1997, 16, 4696. (b) Rieger, A. L.; Carpenter, G. B.; Rieger, P. H. Organometallics 1993, 12, 842. (c) Brown, M. P.; Puddephatt, R. J.; Upton, C. E. J. Chem. Soc., Dalton Trans. 1974, 2457. (d) Duddel, D. A.; Evans, J. G..; Goggin, P. L.; Goodfellow, R. J.; Rest, A. J.; Smith, J. G. J. Chem. Soc. A 1969, 2134.

Stable trans-Bis(P,Si-chelate)metal Complexes

Organometallics, Vol. 23, No. 2, 2004 205

Table 1. NMR Spectroscopic Data (δ) for Compounds 2, 4, 5, 8, 10, and 12a compd 2a 2b

2c 4a 4b

5a

5b

5c 8a 8b 8c 10a

10b

12a 12b 12c

a

1H

SiMe2, 2JSi-H ) 4 Hz) PMe2, 2JP-H ) 5 Hz)

0.37 (d. 6H, 1.26 (d. 6H, 4.32 (m, 1H, SiH) 0.37 (dd, 6H, SiMe2, 2JSi-H ) 4 Hz, 3J H-H ) 1 Hz) 1.31 (t, 6H, POCH2Me, 3JH-H ) 7 Hz) 3.98 (q, 4H, POCH2Me) 4.31 (m, 1H, SiH) 0.42 (d, 6H, SiMe2, 2JSi-H ) 6 Hz) 4.50 (m, 1H, SiH) 7.45, 7.46, 7.74, 7.78, 7.82, (m, 10H, PPh) 0.33 (d, 12H, SiMe2, 3JPt-H ) 13 Hz) 1.87 (dt, 12H, PMe2, 3JPt-H ) 35 Hz, 3JP-H ) 3 Hz) 0.40 (d, 12H, SiMe2, 3JPt-H ) 13 Hz) 1.38 (t, 12H, P(OCH2Me)2, 3JH-H ) 7 Hz) 4.19 (m, 8H, P(OCH2Me)2) 0.41 (dd, 12H, SiMe2, 3JPt-H ) 27 Hz, 2J Si-H ) 2 Hz) 1.72 (dd, 12H, PMe2,3JPt-H ) 17 Hz, 2J P-H ) 7 Hz) 0.41 (dd, 12H, SiMe2, 3JPt-H ) 27 Hz, 2J Si-H ) 2 Hz) 1.36 (t, 12H, P(OCH2Me)2, 3JH-H ) 7 Hz), 4.11 (m, 8H, P(OCH2Me)2) 0.56 (dd, 12H, SiMe2, 3JPt-H ) 28 Hz, 2J Si-H ) 2 Hz) 7.12, 7.38 (m, 20H, PPh2) 0.41 (d, 12H, SiMe2, 2JSi-H ) 2 Hz) 1.60 (d, 12H, PMe2, 2JP-H ) 5 Hz) 0.35 (s, 12H, SiMe2) 1.38 (t, 12H, P(OCH2Me)2, 3JH-H ) 7 Hz) 4.10 (m, 8H, P(OCH2Me)2) 0.52 (s, 12H, SiMe2) 7.13, 7.35 (m, 20H, PPh2) -0.75 (ddd, 1H, PtH, 1JPt-H ) 1119 Hz, 2J 2 P-H(trans) ) 164 Hz, JP-H(cis) ) 22 Hz) 0.62 (d, 6H, SiMe2, 3JPt-H ) 32 Hz) 7.11, 7.14, 7.32, 7.78 (m, 25H, PPh) -1.33 (ddd, 1H, PtH, 1JPt-H ) 1122 Hz, 2J 2 P-H(trans) ) 156 Hz, JP-H(cis) ) 20 Hz) 0.65 (d, 6H, SiMe2, 3JPt-H ) 32 Hz, 2J Si-H ) 2 Hz) 4.78 (s, 1H, CHCab) 7.04, 7.36, 7.60 (m, 20H, PPh) 0.60 (s, 12H, SiMe2) 1.27 (d, 12H, PMe2, 2JP-H ) 5 Hz) 0.65 (s, 12H, SiMe2) 1.41 (t, 12H, P(OCH2Me)2, 3JH-H ) 7 Hz) 4.10 (m, 8H, P(OCH2Me)2) 0.72 (s, 12H, SiMe2) 7.40, 7.42, 7.47, 7.49, 7.69, 7.72, 7.75, 7.79 (20H, PPh2)

13C

31P

-2.25 (d, SiMe2, 1JSi-C ) 8 Hz) 15.37 (d, PMe2, 1JP-C ) 17 Hz)

-11.56 (PMe2)

-2.20 (d, SiMe2, 1JSi-C ) 7 Hz)

-91.97 (P(OCH2Me)2)

16.96 (d, PMe2, 3JP-C ) 9 Hz) 64.64 (d, POCH2, 2JP-C ) 31 Hz) -2.13 (d, SiMe2, 1JSi-C ) 10 Hz) 128.70, 129.12, 131.00, 131.41, 132.92, 133.17, 134.93, 135.07, 135.45, 135.59, 135.82 (PPh) 6.43 (d, SiMe2, 2JPt-C ) 44 Hz) 18.09 (d, PMe2, 2JPt-C ) 39 Hz) 6.53 (SiMe2) 16.29 (P(OCH2Me)2) 66.73 (P(OCH2Me)2) 1.99 (d, SiMe2, 2JPt-C ) 229 Hz)

17.46 (PPh2) 48.60 (d, PMe2, 1JPt-P ) 2702 Hz) 164.80 (d, P(OCH2Me)2, 1J Pt-P ) 4049 Hz) 55.05 (d, PMe2, 1JPt-P ) 1626 Hz)

19.00 (PMe2) 1.96 (SiMe2) 25.82 (P(OCH2Me)2) 47.85 (P(OCH2Me)2) 6.56 (d, SiMe2, 2JPt-C ) 111 Hz) 128.03, 128.17, 128.52, 131.79, 134.86 (PPh2) 5.72 (SiMe2) 21.62 (PMe2) 5.40 (SiMe2) 16.42 (P(OCH2Me)2), 66.58 (P(OCH2Me)2) 6.32 (SiMe2) 128.13, 128.20, 128.27, 131.56, 134.51, 134.62, 134.72 (PPh2) 7.31 (dd, SiMe2, 2JPt-C ) 107 Hz, 1JSi-C ) 6 Hz) 127.71, 128.16, 129.89, 131.38, 133.96, 135.76 (m, PPh) 7.07 (dd, SiMe2, 2JPt-C ) 106 Hz, 1JSi-C ) 7 Hz) 127.91, 131.60, 134.97, 135.17, 135.24, 135.43 (m, PPh) 2.87 (SiMe2) 15.54 (PMe2) 5.40 (SiMe2) 16.42 (P(OCH2Me)2) 66.58 (P(OCH2Me)2) 2.13 (SiMe2) 128.77, 129.21, 130.59, 131.11, 133.71, 133.88, 134.68, 135.01, 135.50 (PPh2)

-71.50 (d, P(OCH2Me)2, 1J Pt-P

) 2492 Hz)

70.99 (d, PPh2, 1JPt-P ) 1814 Hz) 41.68 (PMe2) 171.26 (P(OCH2Me)2) 62.37 (PPh2) 34.65 (d, PPh3, 1JPt-P ) 1747 Hz) 80.99 (d, PPh2, 1JPt-P ) 2660 Hz) 64.32 (d, (Cab)PPh2, 1J Pt-P ) 1756 Hz) 77.27 (d, PPh3, 1JPt-P ) 2744 Hz) -17.60 (PMe2) -88.45 (P(OCH2Me)2) 11.50 (PPh2)

CDCl3 was used as the solvent, and the chemical shifts are reported relative to the residual H of the solvent.

strong trans influence of the silyl group. The increased stability of the trans products 4a,b is most likely a consequence of the formation of the two bis(chelate) rings imposed by the bulky o-carborane backbones. Initial attempts to isomerize the trans isomers were unsuccessful. The dissolution of trans-4a,b in toluene and heating of the solutions for 1 day at 110 °C resulted in no changes in trans-4a,b. However, in the presence of dimethyl acetylenedicarboxylate (DMAD) at 110 °C, the trans isomers 4a,b cleanly rearranged to the thermodynamically favored cis isomers, cis-(CabP,Si)2Pt (5a,b), within 1 h (Scheme 4). Interestingly, only the trans isomers 4a,b with activated acetylene such as DMAD can promote the isomerization to the cis form, indicating that there are elec-

tronic influences on the course of the reaction. The isomerization of toluene solutions of trans-4a,b (0.20 mmol) and DMAD (1.0 mmol) at 110 °C for 1 h afforded high yields of cis-5a,b (92-96%) as colorless solids. The 1H and 31P{1H} NMR spectra of cis-5a,b in CDCl3 contain signals of their isomers generated in the solution. The spectra were assigned to cis-(CabP,Si)2Pt (5a,b) on the basis of a 1JPt-P value (1626-2492 Hz) smaller than that exhibited by trans-4a,b (1JPt-P ) 2702-4049 Hz) and because of their similarity to those values previously reported for cis-Pt(SiR3)2(PR′3)2 type complexes.9 The solution NMR spectral data of the cis-5a,b complexes are unambiguous and consistent with the cis geometry found in the crystal structure of cis-5a. Figure 2 depicts the molecular structure of 5a, which has a

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Lee et al.

Figure 1. Molecular structure of trans-4a with thermal ellipsoids drawn at the 30% level.

Figure 2. Molecular structure of cis-5a with thermal ellipsoids drawn at the 30% level.

Scheme 4. Trans to Cis Isomerization of Bis(chelate)platinum Metal Complexesa

the reaction of Ph2PCH2CH2SiHR1R2 (R1, R2 ) H, Me, Ph) with Pt(cod)2.11 It is well established that the hydrosilylation of low-valent metal complexes by the functional silane PPh2CH2CH2SiMe2H provides access to a family of new bis(chelate) derivatives of the silyl PPh2CH2CH2SiMe2-, in which the silicon-transitionmetal bond is supported by phosphine coordination to the metal. The isomerization of cis-(PhMe2P)2Pt(SiMe2Ph)2 into trans-(PhMe2P)2Pt(SiMe2Ph)2 catalyzed by di- and oligosilanes has been recently reported.7a In particular, similar trans-cis isomerization has been observed in the work of Kim et al.7b on trans-Pt(SiHPh2)2(PMe3)2, which is readily converted upon dissolution in THF or CD2Cl2 into the thermodynamically more stable cis isomer. In contrast, trans-4a,b are more robust and do not undergo trans-cis isomerization as readily in solution. The increased stability of the trans products 4a,b is most likely a consequence of the formation of the two bis-chelate rings imposed by the bulky o-carborane backbones. In addition, we have used density functional calculations to study both trans-4a and cis-5a bis(chelate) platinum complexes and found that the trans4a structure is preferred over that of cis-5a by 1.66 kcal/ mol (Figure 3). Other group 10 metal complexes, such as Pd2(dba)3 (6) and Pt(cod)2 (9), were tested for the chelate-assisted oxidative addition of 2, as shown in Schemes 5 and 6 and in Table 2. The reaction of 6 with 2a results in the bis(chelates) trans-(CabP,Si)2Pd (7a) and cis-(CabP,Si)2Pd (8a), in 34% yield with a trans:cis ratio of 2:1. On the other hand, the reaction of 6 with 2b,c is cisselective, producing 60-65% yields of cis-(CabP,Si)2Pd (8b,c). These results indicate that the stereoselectivity of the reaction is dependent on the electronic and steric characteristics of the ligands around the metal center.

a

Conditions: (i) DMAD, toluene, 110 °C.

distorted-square-planar coordination around the metal center with two dimethylsilyl units in the cis positions. The Pt-P and Pt-Si distances of the terminal ligands and the chelated (phosphino-o-carboranyl)silyl ligand do not differ significantly, and the Pt-Si distances are comparable to those found in other Pt(II) silyl complexes.10 Similar cyclic bis[((diphenylphosphino)ethyl)diorganosilyl]platinum complexes have been prepared using (9) (a) Yamashita, H.; Tanaka, M.; Goto, M. Organometallics 1992, 11, 3227. (b) Holmes-Smith, R. D.; Stobart, S. R.; Cameron, T. S.; Jochem, K. J. Chem. Soc., Chem. Commun. 1981, 937. (c) Chatt, J.; Eaborn, C.; Ibekwe, S. D.; Kapoor, P. N. J. Chem. Soc. A 1970, 1343. (10) Recent articles on silylplatinum complexes: (a) Ozawa, F.; Hikida, T. Organometallics 1996, 15, 4501. (b) Goikhman, R.; Aizenberg, M.; Shimon, L. J. W.; Milstein, D. J. Am. Chem. Soc. 1996, 118, 10894. (c) Levy, C. J.; Vittal, J. J.; Puddephatt, R. J. Organometallics 1996, 15, 2108. (d) Levy, C. J.; Puddephatt, R. J.; Vittal, J. J. Organometallics 1994, 13, 1559. (e) Ozawa, F.; Hikida, T.; Hayashi, T. J. Am. Chem. Soc. 1994, 116, 2844. (f) Yamashita, H.; Tanaka, M.; Goto, M. Organometallics 1993, 12, 988. (g) Sakaki, S.; Ieki, M. J. Am. Chem. Soc. 1993, 115, 2373. (h) Braunstein, P.; Knorr, M.; Hirle, B.; Reinhard, G.; Schubert, U. Angew. Chem., Int. Ed. Engl. 1992, 31, 1583. (i) Tanaka, M.; Uchimaru, Y.; Lautenschlager, H.-J. J. Organomet. Chem. 1992, 428, 1. (j) Yamashita, H.; Tanaka, M.; Goto, M. Organometallics 1992, 11, 3227. (k) Heyn, R. H.; Tilley, T. D. J. Am. Chem. Soc. 1992, 114, 1917. (l) Grundy, S. L.; Holmes-Smith, R. D.; Stobart, S. R.; Williams, M. A. Inorg. Chem. 1991, 30, 3333. (m) Pham, E. K.; West, R. Organometallics 1990, 9, 1517.

(11) (a) Grundy, S. L.; Holmes-Smith, R. D.; Stobart, S. R.; Williams, M. A. Inorg. Chem. 1991, 30, 3333. (b) Holmes-Smith, R. D.; Stobart, S. R.; Cameron, T. S.; Jochem, K. J. Chem. Soc., Chem. Commun. 1981, 937.

Stable trans-Bis(P,Si-chelate)metal Complexes

Organometallics, Vol. 23, No. 2, 2004 207

Figure 3. Energy profiles of trans-4a and cis-5a. Scheme 6. Reaction of Pt(cod)2 (9) with (Phosphino-o-carboranyl)silane (2)a

Scheme 5. Reaction of Pd2(dba)3 (6) with (Phosphino-o-carboranyl)silane (2)a

a

a Conditions: (i) 2 2a, toluene, 25 °C; (ii) 2 2b,c, toluene, 25 °C.

The 1H, 13C, and 31P NMR spectral data for the resulting palladium species trans-7 and cis-8 are summarized in Table 1. All the complexes exhibit NMR spectra that are in agreement with their proposed

Conditions: (i) 2 2, toluene, 25 °C.

structures. In addition, we established the structure of cis-8c unambiguously using single-crystal X-ray analysis, as shown in Figure 4. The crystallographic data and processing parameters are given in Table 3; selected bond distances and angles are given in Tables 4 and 5, respectively. Complex cis-8c has a slightly distorted square-planar geometry with a cis arrangement of the two silicon atoms. The equatorial plane is defined by the Pd(1), P(1), P(2), Si(1), and Si(2) atoms and is relatively planar with an average atomic displacement of 0.0426 Å. The two five-membered metallacycles (Pd(1), C(1), C(2), Si(1), P(1); Pd(1), C(17), C(18), Si(2), P(2)) are twisted with respect to each other at a dihedral angle of 8.8(1)°. These metallacycles are also twisted with respect to the equatorial plane, with dihedral

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Lee et al.

Table 2. Chelate-Assisted Oxidative Addition Reaction of 2

entry

substrate

MLn

1 2 3 4 5 6 7 8

2a 2b 2a 2b 2c 2a 2b 2c

(PPh3)2Pt(C2H4) (3) (PPh3)2Pt(C2H4) (3) Pd2(dba)3 (6) Pd2(dba)3 (6) Pd2(dba)3 (6) Pt(cod)2 (9) Pt(cod)2 (9) Pt(cod)2 (9)

(CabSi,P)2M trans cis 4a 4b 2 7ac none none 2 4ac 1 4bc 1 4cc

none none 1 8ac 8b 8c 1 5ac 1 5bc 6 5cc

yield (%)b 73 76 34 65 60 59 51 43

a Conditions: 0.60 mmol of 2, benzene, 25 °C, Ar atm. b Yields determined after recrystallization or chromatography. c Stereoisomeric ratios were determined by NMR.

Figure 4. Molecular structure of cis-8c with thermal ellipsoids drawn at the 30% level.

angles of 17.1(1) and 8.4(1)°, respectively. The Pd-Si bond distance (2.358(2)-2.361(2) Å) of cis-8c is in agreement with the values observed for Pd(SiMe2CH2CH2PPh2)2 (2.367(1) Å)12 and (dcpe)Pd(SiHMe2)2 (2.3561 Å)13 and for analogous complexes (2.34-2.41 Å).14 The 1H, 13C, and 31P NMR spectra of cis-8c are consistent with the structure determined by X-ray crystallography. An 1H NMR signal ascribable to the Pd-SiMe2 moiety was observed at 0.52 ppm. The 31P NMR signal had clearly shifted, moving from a value of 17.46 ppm for 2c to 62.37 ppm for cis-8c. Bis(silyl)palladium complexes have already been reported by Ito et al.15 and have been implicated as key (12) Murakami, M.; Yoshida, T.; Ito, Y. Organometallics 1994, 13, 2900. (13) Pan, Y.; Mague, J. T.; Fink, M. J. Organometallics 1992, 11, 3495. (14) (a) Suginome, M.; Oike, H.; Park, S.; Ito, Y. Bull. Chem. Soc. Jpn. 1996, 69, 289. (b) Michalczyk, M. J.; Tilley, T. D. J. Am. Chem. Soc. 1992, 114, 1917. (c) Holmes-Smith, R. D.; Stobart, S. R.; Cameron, T. S.; Jochem, K. J. Chem. Soc., Chem. Commun. 1981, 937.

Figure 5. Molecular structure of cis-5c with thermal ellipsoids drawn at the 30% level.

intermediates in the Pd-catalyzed bis-silylation of alkynes.16 This suggested that an investigation of the reactivity of cis-(CabP,Si)2Pd (8b,c) with alkynes might be useful. However, these cis isomers are inactive toward bis-silylation, indicating that the bulky o-carborane backbone confers some additional strength to the Pd-Si bond. In contrast, the route using Pt(cod)2 (9) does not exhibit sufficient stereoselectivity. The use of 2a gives 4a and 5a with a trans:cis ratio of 2:1, whereas 2b yields 4b and 5b with a trans:cis ratio of 1:1. On the other hand, when 9 is reacted with 2c, the major product formed is the cis isomer 5c, as well as a minor amount of the trans isomer 4c. A cis arrangement of the chelated ligands in complex 5c is indicated by 1JPt-P ) 1814 Hz, which is close to that reported for cis-[Pt(PPh2CH2CH2SiMe2)2] (1608 Hz), which has mutually cis phosphines.3b This conclusion was corroborated by the results of an X-ray crystal structure determination (Figure 5). The four bulky groups bonded to the platinum atom are accommodated in the square plane by distortion of the bond angles centered at Pt. The Pt-P and Pt-Si distances of the terminal ligands and the chelated (phosphino-o-carboranyl)silyl ligand do not differ significantly, and the Pt-Si distances are comparable to those in other Pt(II) silyl complexes.10 Although a wide range of kinetically stabilized trans isomers can be obtained under mild conditions, the stereochemistry of the reaction nevertheless appears to be dependent on which silane and metal complex are employed. For example, changing the alkyl substituent on the phosphorus center of the silane to one that is more electron donating and less sterically demanding, as is the case in 2a, exclusively leads to the trans isomer. Increased stereoselectivity is also found for metal complexes such as 3 and 6; whether the trans or (15) (a) Murakami, M.; Yoshida, T.; Kawanami, S.; Ito, Y. J. Am. Chem. Soc. 1995, 117, 6408. (b) Murakami, M.; Yoshida, T.; Ito, Y. Organometallics 1994, 13, 2900. (16) (a) Murakami, M.; Suginome, M.; Fujimoto, K.; Ito, Y. Angew. Chem., Int. Ed. Engl. 1993, 32, 1473. (b) Obora, Y.; Tsuji, Y.; Kawamura, T. Organometallics 1993, 12, 2853 and references therein.

Stable trans-Bis(P,Si-chelate)metal Complexes

Organometallics, Vol. 23, No. 2, 2004 209

Table 3. X-ray Crystallographic Data and Processing Parameters for Compounds 4a, 5a,c, 8c, and 10a,c 4a formula fw cryst class space group Z cell constants a, Å b, Å c, Å R, deg β, deg γ, deg V, Å3 µ, mm-1 cryst size, mm dcalcd, g/cm3 F(000) radiation θ range, deg h, k, l collected no. of rflns measd no. of unique rflns no. of rflns used in refinement (I > 2σ(I)) no. of params R1a wR2a GOF

5a

5c

8c

10a

10b

B10C19H52Si2P2Pt 810.03 triclinic P1 h 1

B20C12H44Si2P2Pt 717.88 monoclinic P21/n 4

B20C32H52Si2P2Pt 966.15 monoclinic P21/c 4

B20C32H52Si2P2Pd 877.46 monoclinic P21/c 4

B10C34H42SiP2Pt 843.90 monoclinic P21/c 4

B20C30H48SiP2Pt 910.00 triclinic P1 h 2

8.6548(3) 11.0158(8) 11.0685(7) 104.516(5) 106.106(4) 98.033(4) 956.3(1) 3.830 0.3 × 0.45 × 0.45 1.413 400

9.5000(9) 22.673(1) 14.6629(6)

11.8496(8) 15.686(2) 23.404(1)

11.880 (1) 15.645 (1) 23.405 (2)

13.0608(6) 18.296(1) 16.3890(7)

90.564(5)

93.792(5)

93.557(7)

103.262(4)

3141.8(4) 4.652 0.2 × 0.2 × 0.3 1.518 1408

3811.9(3) 3.821 0.3 × 0.3 × 0.3 1.470 1672

1.96-25.97 0-10, +13 to -13, +13 to -13, 4060 3758 3709

1.66-25.97 0-11, 0-27, -18 to +18 6558 6142 5079

4340.7(6) 4341.9(6) 3.389 0.583 0.35 × 0.5 × 0.5 0.4 × 0.4 × 0.5 1.478 1.342 1920 1792 Mo KR (λ ) 0.7107) 1.56-25.98 1.57-25.97 0-14, 0-19, 0-14, 0-19, -28 to +28 -28 to +28 8974 8886 8510 8459 6676 5760

11.9428(6) 12.8016(6) 13.681(2) 96.647(8) 95.711(8) 92.512(4) 2064.0(4) 3.531 0.2 × 0.3 × 0.3 1.464 900

1.60-25.97 -16 to +16, 0-22, 0-20 7793 7481 4919

1.51-25.97 0-14, 0-15, -16 to +16 8535 8086 7320

199 0.0288 0.0732 1.078

362 0.0342 0.1539 1.236

558 0.0298 0.0699 1.076

471 0.0389 0.1143 0.710

534 0.0311 0.1317 1.116

558 0.0614 0.2080 1.057

a R1 ) ∑F - F (based on reflections with F 2 > 2σF2). wR2 ) [∑[w(F 2 - F 2)2]/∑[w(F 2)2]]1/2; w ) 1/[σ2(F 2) + (0.095P)2]; P ) [max(F 2, o c o o c o o o 0) + 2Fc2]/3 (also with Fo2 > 2σF2).

Table 4. Selected Interatomic Distances (Å) for Compounds 4a, 5a,c, 8c, and 10a,c Pt(1)-P(1) P(1)-C(4) Si(1)-C(2) C(10)-C(12)#2

2.251(1) 1.824(5) 1.950(4) 1.42(2)

Pt(1)-Si(1) P(1)-C(1) C(1)-C(2) C(12)-C(10)#2

2.408(1) 1.869(4) 1.676(6) 1.42(2)

Compound 4a Pt(1)-P(1)#1 2.251(1) P(1)-C(4) 1.824(5) C(10)-C(11) 1.23(2)

Pt(1)-Si(1)#1 Si(1)-C(5) C(11)-C(12)

2.408(1) 1.878(5) 1.34(2)

P(1)-C(3) Si(1)-C(6) C(12)-C(13)

1.813(5) 1.888(5) 1.17(3)

Compound 5a Pt(1)-Si(1) 2.3492(2) P(2)-C(9) 1.785(8) Si(1)-C(2) 1.928(7)

Pt(1)-Si(2) P(2)-C(10) Si(2)-C(11)

2.3532(2) 1.817(9) 1.859(9)

P(1)-C(3) P(2)-C(7) Si(2)-C(12)

1.811(9) 1.851(8) 1.866(9)

2.362(1) 1.950(5) 1.825(4)

Compound 5c Pt(1)-P(2) 2.337(1) P(2)-C(18) 1.882(4) Si(2)-C(19) 1.879(5)

Pt(1)-Si(2) Si(1)-C(3) Si(2)-C(20)

2.361(1) 1.871(5) 1.882(5)

Si(1)-C(1) Si(1)-C(4) P(2)-C(21)

1.953(5) 1.889(5) 1.823(4)

Pd(1)-Si(1) Si(2)-C(17) P(1)-C(11)

2.361(2) 1.951(8) 1.821(7)

Compound 8c Pd(1)-P(2) 2.357(2) P(2)-C(18) 1.881(7) Si(2)-C(19) 1.889(8)

Pd(1)-Si(2) Si(1)-C(3) Si(2)-C(20)

2.358(2) 1.875(9) 1.874(8)

Si(1)-C(1) Si(1)-C(4) P(2)-C(21)

1.959(7) 1.887(8) 1.822(7)

Compound 10a Pt(1)-P(2) 2.353(2) P(2)-C(23) 1.830(6)

P(1)-C(3) P(2)-C(29)

P(1)-C(9) Si(1)-C(16)

1.832(6) 1.861(2)

P(1)-C(9) Si(1)-C(16)

1.824(6) 1.877(9)

Pt(1)-P(1) P(1)-C(4) Si(1)-C(5) Si(2)-C(8)

2.3122(2) 1.818(9) 1.874(8) 1.947(7)

Pt(1)-P(2) P(1)-C(1) Si(1)-C(6) C(1)-C(2)

Pt(1)-P(1) P(1)-C(2) P(1)-C(5) P(2)-C(27)

2.338(1) 1.887(4) 1.812(4) 1.829(4)

Pt(1)-Si(1) Si(2)-C(17) P(1)-C(11)

Pd(1)-P(1) P(1)-C(2) P(1)-C(5) P(2)-C(27)

2.363(2) 1.890(7) 1.814(7) 1.832(8)

2.3085(2) 1.884(8) 1.886(9) 1.667(1)

Pt(1)-P(1) P(1)-C(1) Si(1)-C(15)

2.271(2) 1.907(6) 1.875(2)

Pt(1)-Si(1) P(2)-C(17) Si(1)-C(2)

2.307(2) 1.823(6) 1.942(7)

Pt(1)-P(1) P(1)-C(1) Si(1)-C(15)

2.2804(1) 1.864(6) 1.877(1)

Pt(1)-Si(1) P(2)-C(19) Si(1)-C(2)

2.3228(2) 1.826(6) 1.941(7)

Compound 10b Pt(1)-P(2) 2.3642(1) P(2)-C(25) 1.837(7)

the cis isomer is produced depends on the steric bulk of the phosphine substituent. Of the silanes we studied, 2c failed to produce an appreciable amount of trans-bis(chelates). Interestingly, 3 reacted with 2c by displacement of the ethylene ligand and the oxidative addition of the Si-H bond17 to generate the chelating (phosphinoalkyl)silane stabilized

P(1)-C(3) P(2)-C(17)

1.808(6) 1.837(6)

1.822(6) 1.896(6)

cis-[(hydridosilyl)PtII] complex18 (CabP,Si)Pt(H)(PPh3) (10a). In contrast, such a hydridosilyl mono(chelate) was (17) (a) Schubert, U. In Progress in Organosilicon Chemistry; Marciniec, B., Chojnowski, J., Eds.; Gordon and Breach: Lausanne, Switzerland, 1995; pp 287-307. (b) Sakaki, S.; Ieki, M. J. Am. Chem. Soc. 1993, 115, 2373. (c) Schubert, U. Adv. Organomet. Chem. 1990, 30, 151.

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Table 5. Selected Interatomic Angles (deg) for Compounds 4a, 5a,c, 8c, and 10a,c P(1)-Pt(1)-P(1)#1 C(4)-P(1)-C(1) P(1)-Pt(1)-Si(1)#1 C(1)-P(1)-Pt(1)

180.0 104.2(2) 92.12(4) 113.3(1)

P(1)#1-Pt(1)-Si(1) C(4)-P(1)-Pt(1) Si(1)-Pt(1)-Si(1)#1 C(5)-Si(1)-Pt(1)

Compound 4a 92.12(4) P(1)#1-Pt(1)-Si(1)#1 113.9(2) C(6)-Si(1)-Pt(1) 180.0 C(3)-P(1)-C(1) 118.9(2) C(2)-Si(1)-Pt(1)

P(2)-Pt(1)-P(1) P(1)-Pt(1)-Si(2) C(4)-P(1)-C(1) C(9)-P(2)-C(10) C(10)-P(2)-Pt(1) C(2)-Si(1)-Pt(1)

97.36(7) 174.57(7) 104.1(4) 102.3(5) 115.8(3) 106.9(2)

P(2)-Pt(1)-Si(1) Si(1)-Pt(1)-Si(2) C(3)-P(1)-Pt(1) C(9)-P(2)-C(7) C(7)-P(2)-Pt(1) C(11)-Si(2)-Pt(1)

Compound 5a 175.11(7) P(1)-Pt(1)-Si(1) 87.61(7) C(3)-P(1)-C(4) 118.7(4) C(4)-P(1)-Pt(1) 101.7(4) C(10)-P(2)-C(7) 111.5(2) C(5)-Si(1)-Pt(1) 119.3(3) C(12)-Si(2)-Pt(1)

87.88(4) 117.6(2) 101.2(2) 106.54(1)

C(3)-P(1)-C(4) P(1)-Pt(1)-Si(1) C(3)-P(1)-Pt(1)

103.9(3) 87.88(4) 118.5(2)

87.03(7) 104.0(5) 114.9(4) 105.3(4) 122.2(3) 115.8(3)

P(2)-Pt(1)-Si(2) C(3)-P(1)-C(1) C(1)-P(1)-Pt(1) C(9)-P(2)-Pt(1) C(6)-Si(1)-Pt(1) C(8)-Si(2)-Pt(1)

87.97(7) 101.0(4) 112.3(2) 118.5(3) 113.2(3) 108.1(2)

P(1)-Pt(1)-Si(1) C(2)-P(1)-Pt(1) C(18)-C(17)-Si(2) C(5)-P(1)-C(11)

87.51(4) 110.1(1) 116.2(3) 104.7(2)

Si(2)-Pt(1)-Si(1) C(1)-C(2)-P(1) C(17)-Si(2)-Pt(1) C(21)-P(2)-C(27)

Compound 5c 86.69(4) P(2)-Pt(1)-Si(2) 112.1(3) C(2)-C(1)-Si(1) 107.6(1) C(17)-C(18)-P(2) 111.5(2)

87.76(4) 116.1(3) 112.4(3)

P(2)-Pt(1)-P(1) C(1)-Si(1)-Pt(1) C(18)-P(2)-Pt(1)

98.00(4) 105.9(1) 112.1(1)

P(1)-Pd(1)-Si(1) Pd(1)-P(1)-C(2) Si(2)-C(17)-C(18) C(5)-P(1)-C(11)

88.08(7) 109.9(2) 117.2(4) 104.2(3)

Si(1)-Pd(1)-Si(2) P(1)-C(2)-C(1) Pd(1)-Si(2)-C(17) C(21)-P(2)-C(27)

Compound 8c 84.71(7) Si(2)-Pd(1)-P(2) 111.5(4) C(2)-C(1)-Si(1) 107.0(2) C(17)-C(18)-P(2) 110.8(3)

88.01(7) 117.6(4) 112.1(4)

P(1)-Pd(1)-P(2) C(1)-Si(1)-Pd(1) C(18)-P(2)-Pd(1)

99.11(6) 105.1(2) 111.8(2)

P(1)-Pt(1)-Si(1) C(3)-P(1)-C(1) C(1)-P(1)-Pt(1) C(29)-P(2)-Pt(1)

90.91(7) 101.7(3) 110.7(2) 116.0(2)

P(1)-Pt(1)-P(2) C(9)-P(1)-C(1) C(17)-P(2)-C(23) C(16)-Si(1)-Pt(1)

Compound 10a 105.18(5) Si(1)-Pt(1)-P(2) 105.3(3) C(3)-P(1)-Pt(1) 105.4(3) C(17)-P(2)-Pt(1) 114.8(4) C(15)-Si(1)-Pt(1)

C(3)-P(1)-C(9) C(9)-P(1)-Pt(1) C(23)-P(2)-Pt(1) C(2)-Si(1)-Pt(1)

105.6(3) 117.6(2) 117.2(2) 107.7(2)

P(1)-Pt(1)-Si(1) C(3)-P(1)-C(1) C(1)-P(1)-Pt(1) C(19)-P(2)-Pt(1) C(2)-Si(1)-Pt(1)

91.16(6) 104.7(3) 110.43(2) 116.0(2) 106.82(2)

P(1)-Pt(1)-P(2) C(9)-P(1)-C(1) C(19)-P(2)-C(25) C(25)-P(2)-Pt(1)

Compound 10b 99.34(5) Si(1)-Pt(1)-P(2) 101.6(2) C(3)-P(1)-Pt(1) 107.8(3) C(16)-Si(1)-Pt(1) 107.4(2) C(17)-P(2)-Pt(1)

not observed for the oxidative addition of analogous (phosphinoalkyl)silanes.3 Thus, as can be seen in Scheme 7, an increase in the steric bulk of the phosphine substituent appears to retard the second oxidative addition, as manifested in the higher yields obtained for 10a. Given this steric constraint, we next turned to determining whether high yields of the mono(chelate) could be obtained if 2c was reacted with 9 in the presence of the bulky ancillary o-carboranylphosphine ligand (Cab)PPh2. Indeed, 2c was found to cleanly react with 9 and (Cab)PPh2 to provide the sterically encumbered cis-[(hydridosilyl)PtII] complex (CabP,Si)Pt(H)[(Cab)PPh2] (10b), which is stable well above 100 °C. The structures of 10a,b agree with the conclusions from the solution NMR data that the atoms are arranged in a square-pyramidal framework. The hydride ligand was not located by X-ray diffraction; however, its presence was confirmed by the appearance of its 1H NMR resonance at around δ -1 (1JPtH ≈ 1119 Hz). Figure 6 shows the structure of 10a as determined by X-ray crystallography. The coordination geometry around the Pt center of the complexes can be rationalized as a distorted square plane containing two PPh2R (R ) Ph, o-carborane) ligands at mutually cis positions and a dimethylsilyl unit and a hydrido ligand coordinated to the Pt(II) center. The Pt-Si bond distances (2.307(2) Å) are in the range previously reported for (18) (a) Mullica, D. F.; Sappenfield, E. L.; Hampden-Smith, M. J. Polyhedron 1991, 10, 867. (b) Clark, H. C.; Hampden-Smith, M. J. Coord. Chem. Rev. 1987, 79, 229. (c) Clark, H. C.; Hampden-Smith, M. J. J. Am. Chem. Soc. 1986, 108, 3829. (d) Azizian, H.; Dixon, K. R.; Eaborn, C.; Pidcock, A.; Shuaib, N. M.; Vinaixa, J. J. Chem. Soc., Chem. Commun. 1982, 1020.

163.81(7) 114.4(2) 111.7(2) 116.3(4) 163.46(7) 112.30(2) 110.7(4) 117.56(2)

C(3)-P(1)-C(9) C(9)-P(1)-Pt(1) C(25)-P(2)-C(17) C(15)-Si(1)-Pt(1)

110.2(3) 116.46(2) 104.3(3) 121.2(4)

Scheme 7. Preparation of cis-[(hydridosilyl)PtII] Complexes 10a,ba

a Conditions: (i) (PPh3)2Pt(C2H4) (3), toluene, 25 °C; (ii) Pt(cod)2 (9), (Cab)PPh2, toluene, 25 °C.

silylplatinum complexes (2.34-2.40 Å).10 The Pt(1)-P(2) bond (2.353(2) Å) is longer than the Pt(1)-P(1) bond (2.271(2) Å), indicating that the trans influence of the dimethylsilyl ligand is more significant than that of the

Stable trans-Bis(P,Si-chelate)metal Complexes

Figure 6. Molecular structure of 10a with thermal ellipsoids drawn at the 30% level.

Figure 7. Molecular structure of 10b with thermal ellipsoids drawn at the 30% level.

hydride. An analogous PPh3-coordinated complex, cis[Pt(H)(SiPh3)(PPh3)2], also has dissimilar Pt-P bonds (2.332(2) and 2.298(3) Å), which arises because the trans influence of the SiPh3 ligand is greater than that of the hydride.19 Serious deviation of the bond angles around the metal center from the ideal 90 or 180° can be attributed to the more severe steric repulsion between the silyl and phosphine ligands than exists between the hydride and other ligands. A similar distorted-square-planar arrangement about the platinum atom is found in (CabP,Si)Pt(H)[(PPh2)C2B10H11] (10b) (Figure 7). The platinum atom is bonded to a hydrogen atom, two phosphorus atoms, and a silicon atom. The atoms of the first platinum coordination sphere display major deviations from planarity (mean (19) Latif, L. A.; Eaborn, C.; Pidcock, A. P.; Weng, N. S. J. Organomet. Chem. 1994, 474, 217.

Organometallics, Vol. 23, No. 2, 2004 211

deviation of -1.004(2) Å), probably as a result of the maximal space requirement for the bulky o-carboranyl phosphine ligand. It has been established that the greater trans influence of the dimethylsilyl group compared with that of the hydride means that the Pt(1)-P(2) bond is longer than the Pt(1)-P(1) bond.19 The measured Pt(1)-Si(1) distance is comparable to other previously published values.10 The 1H NMR spectrum of 10a contains a signal due to the hydride ligand at δ -0.75 as a doublet of doublets flanked with satellites caused by the 195Pt metal center (1JPt-H ) 1119 Hz). The large difference between the two 2JP-H values (164 and 22 Hz) indicates that the phosphine ligands occupy mutually cis positions. The 31P{1H} NMR spectrum contains signals at δ 80.99 and 34.65 with 1JPt-P values of 2660 and 1747 Hz, respectively. The latter is assigned to the phosphine ligand at the trans position of the dimethylsilyl ligands on the basis of a comparison of the 1J values with those of already reported silylplatinum complexes.10 This assignment is consistent with the relative magnitude of the trans influence of SiMe3 and H ligands observed in the crystallographic results. Complex 10b gives rise to similar NMR spectra. Although several cis-Pt(H)(SiAr3)(PPh3)2 type complexes have already been prepared from the reaction of HSiAr3 with Pt(PPh3)4 and with 3,19,20 there have been no reports of similar P,Si-chelate complexes. The preferential cis configuration of 10a,b can be attributed to the fact that the trans influence of both the dimethylsilyl and hydride ligands is much greater than that of PPh3, rendering the trans structure unstable. Preparation of the Linear Bis(phosphino)disilane 1,2-Bis(1-PR2-o-carboranyl)-1,1,2,2-tetramethyl1,2-disilane (R ) Me (12a), OEt (12b), Ph (12c)). The bis(phosphino)disilanes (12) were readily prepared from the dilithium salt of the bis(o-carboranyl)disilanes (11) with 2 equiv of PR2Cl (R ) Me (a), OEt (b), Ph (c)). The dilithium salt of bis(o-carboranyl)disilane generated by the reaction of n-BuLi with bis(o-carboranyl)disilane (11) was reacted with dialkylchlorophosphine to give the bis(phosphino)disilanes (12) in 62-70% yield (Scheme 8). The colorless products 12 were crystalline solids that were relatively stable in air and during brief heating to 110-120 °C. The compounds 12 were readily soluble in hexane, toluene, and THF. The 1H, 13C, and 29Si NMR spectra and the elemental analysis of 12 were found to be consistent with the proposed structures. The 29Si NMR chemical shift of -7.2 to -9.0 ppm was shifted downfield from -3.29 ppm for 12a-c. Reaction of 12 with the Palladium Complex Pd2(dba)3 (6). Treatment of the disilanes 12 with Pd2(dba)3 (6) in toluene at room temperature for 30 min resulted in the oxidative addition of the Si-Si bond21 to the palladium center to form the cis-bis(chelate) palladium complexes 8 (Scheme 9). The composition and stereochemistry of 8 were elucidated from their 31P, 1H, and 13C NMR spectra and (20) Azizian, H.; Dixon, K. R.; Eaborn, C.; Pidcock, A.; Shuaib, N. M.; Vinaixa, J. J. Chem. Soc., Chem. Commun. 1982, 1020. (21) (a) Ozawa, F.; Sugawara, M.; Hayashi, T. Organometallics 1994, 13, 3237. (b) Murakami, M.; Yoshida, T.; Ito, Y. Organometallics 1994, 13, 2900. (c) Pan, Y.; Mague, J. T.; Fink, M. J. Organometallics 1992, 11, 3495. (d) Michalczyk, M. J.; Recatto, C. A.; Calabrese, J. C.; Fink, M. J. J. Am. Chem. Soc. 1992, 114, 7955. (e) Heyn, R. H.; Tilley, T. D. J. Am. Chem. Soc. 1992, 114, 1917 and references therein.

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Organometallics, Vol. 23, No. 2, 2004 Scheme 8. Synthesis of Bis(phosphino-o-carboranyl)disilane 12a

Lee et al.

palladium center by replacement of the dba ligands. The reaction is not restricted to the PR2 (R ) Me (2a), OEt (2b)) derivatives; the bulkier bis((diphenylphosphino)o-carboranyl)disilane (2c) also results in the corresponding cis-bis(chelate) 8c after oxidative addition of the Si-Si bond, but only after extended reaction times. Conclusions

a Conditions: (i) 2 BunLi, THF, -78 °C; (ii) 2 PR2Cl, THF, -78 °C.

Scheme 9. Reaction of Pd2(dba)3 (6) with Bis(phosphino-o-carboranyl)disilane 2a

a

Conditions: (i) Pd2(dba)3 (6), toluene, 25 °C.

microanalysis data. Indeed, the NMR spectra of the palladium complexes are in complete agreement with an authentic sample of 8 (vide infra). Although we did not investigate the mechanism in detail, we believe that the phosphine tethers force the Si-Si linkage into the proximity of the metal center by precoordination and this accelerates the oxidative addition. The Si-Si bond then oxidatively adds to the

The use of an o-carboranyl unit as the auxiliary ligand backbone was found to result in an increase in the difference in kinetic stability between the cis and trans isomers, which enabled the isolation and full characterization of 4a,b and 7a, all having trans structures. Thus, the steric bulkiness of the backbone connecting Si to P in these CabP,Si systems engenders steric constraints on the metal center that increase the kinetic stability of the metal chelates. Therefore, our examination of the chelate-assisted oxidative addition reactions of the phosphinosilanes has enabled us to reach the following conclusions. Our success in demonstrating the formation of kinetically stabilized trans-bis(chelates) relies on the following two factors: (1) the bulkiness of the alkyl groups in the phosphinosilanes and (2) the rapid bis-chelation of the phosphinosilanes with a rigid o-carborane backbone. We intend to use these principles as we continue our research into the stereoselectivity of the chelate-assisted oxidative addition reactions of the d10 metal complexes. Experimental Section General Procedures. All manipulations were performed under a dry, oxygen-free nitrogen or argon atmosphere using standard Schlenk techniques or in a Vacuum Atmospheres HE-493 drybox. Diethyl ether, toluene, hexane, and pentane were distilled under nitrogen from sodium/benzophenone. Dichloromethane was dried with CaH2. Benzene-d6 was distilled under nitrogen from sodium and stored in a Schlenk storage flask until needed. CDCl3 was predried under CaH2 and vacuum-transferred. n-BuLi (1.6 M in hexanes) and Pd2(dba)3 (6) were used as received from Aldrich. (PPh3)2Pt(CH2CH2) (3)22 and Pt(cod)2 (9)23 were obtained by literature methods. All 1H (300.1 MHz, measured in CDCl3), 13C (75.4 MHz, measured in CDCl3), 31P (121.4 MHz, measured in CDCl3), and 29Si NMR spectra (59.60 MHz, measured in CDCl3) were recorded on a Varian Mercury-300BB spectrometer unless otherwise stated. 1H and 13C NMR chemical shifts are reported relative to Me4Si and were determined by reference to the residual 1H or 13C solvent peaks. The IR spectra were recorded on a Biorad FTS-165 spectrophotometer. Elemental analyses were performed with a Carlo Erba Instruments CHNS-O EA1108 analyzer. Preparation of (Phosphino-o-carboranyl)silanes 2. A representative procedure is as follows: to a solution of 0.721 g (5.0 mmol) of o-carborane in 20 mL of THF, precooled to -78 °C, was added 3.2 mL of n-butyllithium (1.6 M in hexanes). The reaction mixture was warmed to room temperature and stirred for 2 h, whereupon it was transferred, via cannula, to a suspension of 5.5 mmol (1.1 equiv) of chlorodimethylphosphine in 20 mL of Et2O that was cooled to -78 °C. The resultant yellow mixture was warmed to room temperature and stirred for 2 h. The volatiles were then removed under reduced pressure; the crude mixture was taken up in a minimal amount of Et2O and the solution filtered through a 1 (22) Hartley, F. R. Organomet. Chem. Rev., Sect. A 1970, 6, 119. (23) Spencer, J. L.; Ittel, S. D.; Cushing, M. A., Jr. Inorg. Synth. 1979, 19, 213.

Stable trans-Bis(P,Si-chelate)metal Complexes × 5 cm pad of silica gel on a glass frit. Removal of the volatiles provided the final crude product, which was further crystallized from toluene at -5 °C to provide pure o-carboranyl phosphine 1a as a colorless solid. THF (40 mL) and 4.0 mmol of 1a were added to the reaction flask. The solution was cooled to -78 °C, n-butyllithium (2.6 mL, 4.16 mmol, 1.6 M in hexanes) was added. The solution was warmed to room temperature, whereupon a thick suspension of white precipitate formed. After the reaction mixture had been stirred for 1 h at room temperature, it was again cooled to -78 °C and chlorodimethylsilane (1.1 equiv) was added as a neat liquid. The solution was warmed to -5 °C very slowly (for over 1 h) and stirred at -5 °C for a total of 3 h. The volatiles were then removed under reduced pressure; the crude mixture was taken up in a minimal amount of Et2O and filtered through a 1 × 5 cm pad of silica gel on a glass frit. Removal of the volatiles provided the final crude product, which was further crystallized from toluene at -5 °C to provide pure ((dimethylphosphino)-o-carboranyl)silane (2a) as a colorless waxy solid. Yield: 92% (0.966 g, 3.7 mmol). Anal. Calcd for B10C6H23PSi: C, 27.25; H, 8.77. Found: C, 27.17; H, 8.74. IR (KBr pellet, cm-1): 2966 w, 2909 w (νCH), 2611 s, 2570 w (νBH), 2162 w (νSiH). 2b. A procedure analogous to the preparation of 2a was used, but starting from chlorodiethoxyphosphine (0.861 g, 5.5 mmol) in benzene. Thus, 2b was crystallized from toluene at -5 °C. Yield: 89% (1.148 g, 3.6 mmol). Anal. Calcd for B10C8H27O2PSi: C, 29.61; H, 8.39. Found: C, 29.51; H, 8.36. IR (KBr pellet, cm-1): 2979 w, 2930 w (νCH), 2576 s (νBH), 2165 w (νSiH). 2c. A procedure analogous to the preparation of 2a was used, but starting from chlorodiphenylphosphine (1.214 g, 5.5 mmol) in benzene. Thus, 2c was crystallized from toluene at -5 °C. Yield: 82% (1.209 g, 3.3 mmol). Anal. Calcd for B10C16H27PSi: C, 49.45; H, 7.01. Found: C, 49.26; H, 7.03. IR (KBr pellet, cm-1): 3058 m (νCH), 2570 s (νBH), 2170 m (νSiH). Synthesis of [η2-(phosphino-o-carboranyl)silyl]metal Complexes (CabP,Si)2M. A representative procedure is as follows: toluene (15 mL) was added to a mixture of (PPh3)2Pt(C2H4) (3; 0.224 g, 0.30 mmol) and 2a (0.157 g, 0.60 mmol). The mixture was stirred for 3 h, and then the volatile compound was removed under reduced pressure. Extraction of the residue with toluene (20 mL), followed by concentration of the extract to approximately half its volume and cooling to -5 °C, resulted in crystallization of pure trans-4a. Yield: 73% (0.157 g, 0.22 mmol). Anal. Calcd for B20C12H44P2PtSi2: C, 20.08; H, 6.18. Found: C, 20.12; H, 6.22. Mp: 235-237 °C dec. IR (KBr pellet, cm-1): 2960 m, 2903 w (νCH), 2627 s, 2598 s, 2571 s (νBH). trans-4b. A procedure analogous to the preparation of 4a was used, but starting from 2b (0.193 g, 0.60 mmol) in toluene. Thus, 4b was crystallized from toluene at -5 °C. Yield: 76% (0.184 g, 0.23 mmol). Anal. Calcd for B20C16H52O2P2Si2Pt: C, 23.84; H, 6.5. Found: C, 23.90; H, 6.72. Mp: 200-203 °C dec. IR (KBr pellet, cm-1): 2985 w, 2896 w (νCH), 2611 s, 2566 s (νBH). cis-5a. DMAD (0.12 mL, 1.0 mmol) was added to a solution of trans-4a (0.144 g, 0.20 mmol) in toluene (20 mL), and the mixture was heated for 1 h at 110 °C; conversion into cis-5a, as monitored by 1H NMR spectroscopy, was quantitative. The solvent was removed in vacuo and the resulting solid crystallized from toluene at -5 °C to give 0.138 g (0.19 mmol) of yellow microcrystals (96% yield). Anal. Calcd for B20C12H44P2PtSi2: C, 20.08; H, 6.18. Found: C, 20.15; H, 6.25. Mp: 239241 °C dec. IR (KBr pellet, cm-1): 2956 w, 2924 w, 2852 w (νCH), 2607 s, 2568 s (νBH). cis-5b. A procedure analogous to the preparation of cis-5a was used, but starting from trans-4b (0.161 g, 0.20 mmol) in toluene. Thus, cis-5b was crystallized from toluene at -5 °C. Yield: 92% (0.148 g, 0.184 mmol). Anal. Calcd for B20C16H52O2P2-

Organometallics, Vol. 23, No. 2, 2004 213 Si2Pt: C, 23.84; H, 6.5. Found: C, 23.91; H, 6.69. Mp: 208210 °C dec. IR (KBr pellet, cm-1): 2955 w, 2928 w (νCH), 2577 s (νBH). General Procedure for cis-5c and 8. Toluene (20 mL) was added to a mixture of suitable metal reagent (0.30 mmol) and 2 (0.60 mmol, 2 equiv). The mixture was stirred for 12 h, and then the volatile compounds were removed under reduced pressure. Extraction of the residue with toluene (2 × 15 mL), followed by concentration and chromatography on silica with 1:1 hexane/benzene as eluent gave bis(chelates) as yellow solids. cis-5c. Anal. Calcd for B20C32H52P2Si2Pt: C, 39.78; H, 5.42. Found: C, 39.85; H, 5.52. Mp: 245-248 °C dec. IR (KBr pellet, cm-1): 3060 w, 2957 w, 2900 w (νCH), 2575 s (νBH). cis-8a. Anal. Calcd for B20C12H44P2Si2Pd: C, 22.91; H, 7.05. Found: C, 22.97; H, 7.10. Mp: 225-228 °C dec. IR (KBr pellet, cm-1): 2959 w, 2917 w (νCH), 2606 s, 2561 s (νBH). cis-8b. Anal. Calcd for B20C16H52O2P2Si2Pd: C, 26.79; H, 7.31. Found: C, 26.85; H, 7.42. Mp: 206-208 °C dec. IR (KBr pellet, cm-1): 2992 w (νCH), 2603 s, 2567 s (νBH). cis-8c. Anal. Calcd for B20C32H52P2Si2Pd: C, 43.80; H, 5.97. Found: C, 43.85; H, 6.02. Mp: 225-227 °C dec. IR (KBr pellet, cm-1): 3063 w, 2961 w (νCH), 2604 s, 2563 s (νBH). 10a. A solution containing 3 (0.30 mmol) and 2c (1 equiv) in toluene was stirred for 2 h; conversion into 10a, as monitored by 1H NMR spectroscopy, was quantitative. Removal of the volatile material under reduced pressure gave a light yellow oil, to which toluene (5 mL) was added. Cooling the mixture to -5 °C resulted in precipitation of 10a as colorless microcrystals. Yield: 89% (0.225 g, 0.27 mmol). Anal. Calcd for B10C34H42P2SiPt: C, 48.39; H, 5.02. Found: C, 48.44; H, 5.10. Mp: 198 °C dec. IR (KBr pellet, cm-1): 3051 w, 2957 w (νCH), 2581 s (νBH), 2029 s (νPtH). 10b. To a 20 mL solution containing 9 (0.3 mmol) in toluene were added sequentially 1 equiv of 1c and 1 equiv of 2c, to generate 10b in quantitative yield. The solution was filtered, the volatiles were removed, and the resulting solid was crystallized from toluene to give colorless microcrystals. Yield: 82% (0.224 g, 0.25 mmol). Anal. Calcd for B20C30H48O2P2SiPt: C, 39.6; H, 5.32. Found: C, 40.01; H, 5.48. Mp: 225 °C dec. IR (KBr pellet, cm-1): 3055 w, 3033 w, 2962 w (νCH), 2603 s, 2579 s, 2544 s (νBH), 2034 w, 1992 m (νPtH). Preparation of the Linear Bis(phosphino)disilane 1,2Bis(1-PR 2 -o-carboranyl)-1,1,2,2-tetramethyl-1,2-disilane (R ) Me (12a), OEt (12b), Ph (12c)). A representative procedure is as follows: to a solution of 0.806 g (2.0 mmol) of bis(o-carboranyl)disilane 11 in 20 mL of THF, precooled to -78 °C, was added 2.5 mL of n-butyllithium (1.6 M in hexanes). The reaction mixture was warmed to room temperature and stirred for 2 h, whereupon it was transferred, via cannula, to a suspension of 4.4 mmol (2.2 equiv) of chlorodimethylphosphine in 20 mL of Et2O that was cooled to -78 °C. The resultant yellow mixture was warmed to room temperature and stirred for 2 h. The volatiles were then removed under reduced pressure; the crude mixture was taken up in a minimal amount of Et2O and filtered through a 1 × 5 cm pad of silica gel on a glass frit. Removal of the volatiles provided the final crude product, which was further crystallized from toluene at -5 °C to provide pure bis((dimethylphosphino)-ocarboranyl)disilane (12a) as a colorless solid. Yield: 70% (0.734 g, 1.4 mmol). Anal. Calcd for B20C12H44PSi: C, 27.35; H, 8.42. Found: C, 27.27; H, 8.39. Mp: 225-227 °C. 29Si NMR: δ -9.03. IR (KBr pellet, cm-1): 2982 m, 2910 m (νCH), 2611 s, 2586 s, 2569 s (νBH). 12b. A procedure analogous to the preparation of 12a was used, but starting from chlorodiethoxyphosphine (0.689 g, 4.4 mmol) in benzene. Thus, 12b was crystallized from toluene at -5 °C. Yield: 62% (0.743 g, 1.2 mmol). Anal. Calcd for B20C16H52O2PSi: C, 31.25; H, 8.53. Found: C, 31.35; H, 8.50. Mp: 230-233 °C. 29Si NMR: δ -8.95. IR (KBr pellet, cm-1): 2975 m, 2940 m (νCH), 2601 s, 2577 s, 2568 s (νBH).

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12c. A procedure analogous to the preparation of 12a was used, but starting from chlorodiphenylphosphine (0.971 g, 4.4 mmol) in benzene. Thus, 12c was crystallized from toluene at -5 °C. Yield: 68% (1.046 g, 1.4 mmol). Anal. Calcd for B20C32H52PSi: C, 49.58; H, 6.77. Found: C, 49.41; H, 6.75. Mp: 247-248 °C. 29Si NMR: δ -7.19. IR (KBr pellet, cm-1): 3051 w, 2955 w (νCH), 2611 w, 2575 s (νBH). Reaction of 12 with the Palladium Complex Pd2(dba)3 (6). Representative procedure: Toluene (15 mL) was added to a mixture of Pd2(dba)3 (6; 0.092 g, 0.1 mmol) and 12a (0.115 g, 0.22 mmol). The mixture was stirred for 3 h, and then the volatile compounds were removed under reduced pressure. Extraction of the residue with toluene (20 mL), followed by concentration of the extract to approximately half its volume and cooling to -5 °C, resulted in crystallization of pure cis8a. Yield: 84% (0.107 g, 0.17 mmol). cis-8b. A procedure analogous to the preparation of cis-8a was used, but starting from 12b (0.122 g, 0.22 mmol) in toluene. Thus, cis-8b was crystallized from toluene at -5 °C. Yield: 81% (0.115 g, 0.16 mmol). cis-8c. A procedure analogous to the preparation of cis-8a was used, but starting from 12c (0.170 g, 0.22 mmol) in toluene. Thus, cis-8c was crystallized from toluene at -5 °C. Yield: 86% (0.150 g, 0.17 mmol). Crystal Structure Determination. Crystals of 4a, 5a,c, 8c, and 10a,b were obtained from toluene, sealed in glass capillaries under argon, and mounted on the diffractometer. Data were collected and corrected for Lorentz and polarization effects. Each structure was solved by the application of direct methods using the SHELXS-96 program24a and least-squares refinement using SHELXL-97.24b After anisotropic refinement of all non-H atoms, several H atom positions could be located in difference Fourier maps. These were refined isotropically, while the remaining H atoms were calculated in idealized positions and included in the refinement with fixed atomic contributions. Further detailed information is given in Table 3. Computational Details. Stationary points on the potential energy surface were calculated using the Amsterdam Density Functional (ADF) program, developed by Baerends et al.25,26 and vectorized by Ravenek.27 The numerical integration scheme applied for the calculations was developed by te Velde et al.28,29 The geometry optimization procedure was based on the method due to Versluis and Ziegler.30 The electronic (24) (a) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, A46, 467. (b) Sheldrick, G. M. SHELXL, Program for Crystal Structure Refinement; University of Go¨ttingen, Go¨ttingen, Germany, 1997. (25) Baerends, E. J.; Ellis, D. E.; Ros, P. Chem. Phys. 1973, 2, 41. (26) Baerends, E. J.; Ros, P. Chem. Phys. 1973, 2, 52. (27) Ravenek, W. In Algorithms and Applications on Vector and Parallel Computers; te Riele, H. J. J., Dekker, T. J., van de Horst, H. A., Eds.; Elsevier: Amsterdam, The Netherlands, 1987.

Lee et al. configurations of the molecular systems were described by double-ζ STO basis sets with polarization functions for the H, B, and C atoms, while triple-ζ Slater type basis sets were employed for the Si, P, and Pt atoms.31,32 The 1s electrons of B and C, the 1s-2p electrons of Si and P, and the 1s-4d electrons of Pt were treated as frozen cores. A set of auxiliary33 s, p, d, f, and g STO functions, centered on all nuclei, was used in order to fit the molecular density and the Coulomb and exchange potentials in each SCF cycle. Energy differences were calculated by augmenting the local exchange-correlation potential by Vosko et al.34 with Becke’s35 nonlocal exchange corrections and Perdew’s36 nonlocal correlation corrections (BP86). Geometries were optimized including nonlocal corrections at this level of theory. First-order Pauli scalar relativistic corrections37,38 were added variationally to the total energy for all systems. In view of the fact that all systems investigated in this work show a large HOMO-LUMO gap, a spinrestricted formalism was used for all calculations. No symmetry constraints were used.

Acknowledgment. We are grateful to the Korea Research Foundation (Grant No. KRF-2002-015-CP0200) for their financial support. Supporting Information Available: Crystallographic data (excluding structure factors) for the structures 4a, 5a,c, 8c, and 10a,b reported in this paper and listings giving optimized geometries of the crucial structures (4a and 5a) reported (Cartesian coordinates, in Å); crystallographic data are also available in electronic form as CIF files. This material is available free of charge via the Internet at http://pubs.acs.org. OM034112L (28) te Velde, G.; Baerends, E. J. J. Comput. Chem. 1992, 99, 84. (29) Boerrigter, P. M.; te Velde, G.; Baerends, E. J. Int. J. Quantum Chem. 1988, 33, 87. (30) Versluis, L.; Ziegler, T. J. Chem. Phys. 1988, 88, 322. (31) Snijders, J. G.; Baerends, E. J.; Vernoijs, P. At. Nucl. Data Tables 1982, 26, 483. (32) Vernoijis, P.; Snijders, J. G.; Baerends, E. J. Slater Type Basis Functions for the Whole Periodic System; Internal Report (in Dutch); Department of Theoretical Chemistry, Free University: Amsterdam, The Netherlands, 1981. (33) Krijn, J.; Baerends, E. J. Fit Functions in the HFS Method; Internal Report (in Dutch); Department of Theoretical Chemistry, Free University: Amsterdam, The Netherlands, 1984. (34) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200. (35) Becke, A. Phys. Rev. A 1988, 38, 3098. (36) (a) Perdew, J. P. Phys. Rev. B 1986, 34, 7406. (b) Perdew, J. P. Phys. Rev. B 1986, 33, 8822. (37) Snijders, J. G.; Baerends, E. J. Mol. Phys. 1978, 36, 1789. (38) Snijders, J. G.; Baerends, E. J.; Ros, P. Mol. Phys. 1979, 38, 1909.